Questions: Stoichiometric Calculations: From Balanced Equations
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
In the reaction 2H₂ + O₂ → 2H₂O, if you start with 4.0 mol of H₂ and excess O₂, how many moles of H₂O are produced?
A2.0 mol
B4.0 mol
C8.0 mol
D1.0 mol
The balanced equation shows a 2:2 (i.e., 1:1) mole ratio between H₂ and H₂O. So 4.0 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4.0 mol H₂O. A common error is using the coefficient 2 as a multiplier rather than reading the ratio correctly.
Question 2 True / False
The coefficients in a balanced chemical equation represent mass ratios and can be used to convert grams of one substance directly to grams of another.
TTrue
FFalse
Answer: False
Coefficients represent mole ratios, not mass ratios. Different substances have different molar masses, so a 2:1 mole ratio does not mean a 2:1 mass ratio. The correct pathway always converts grams to moles first, applies the mole ratio, then converts back to grams using the target substance's molar mass.
Question 3 Short Answer
Describe the four-step conversion pathway used to calculate grams of product from grams of a known reactant in a stoichiometry problem.
Think about your answer, then reveal below.
Model answer: Convert grams of the given substance to moles (divide by its molar mass), use the balanced equation's mole ratio to find moles of the target substance, then convert moles of the target to grams (multiply by its molar mass).
Stoichiometry always routes through moles because the balanced equation speaks in moles. Molar mass is the bridge between the mass world (grams) and the counting world (moles). Skipping any step — especially the mole ratio — is the most common source of errors in stoichiometry calculations.